Currently, digital X-ray images are preferably recorded by way of indirect converters including e.g. a CsI scintillator layer deposited onto an a-Si photodetector matrix. Alternatively, direct converters, such as e.g. a-Se, also find application, primarily in applications that require a high resolution, such as mammography. Detectors based on amorphous silicon (indirect conversion) and amorphous selenium (direct conversion) therefore represent the current background art.
The principles underlying direct conversion and indirect conversion are represented schematically in FIG. 1 and FIG. 2, respectively. In the case of direct conversion, an X-ray quantum 1 is absorbed in the semiconductor 2, during which process electron-hole pairs 2a, 2b are generated which then migrate to the electrodes 4 (anode and cathode respectively, e.g. pixel electrodes) and are detected there. In the case of indirect conversion, the X-ray quantum 1 is absorbed in the scintillator 2, which in turn emits radiation 2′ at lower energy (e.g. visible light, UV or IR radiation), which is then detected via a photodetector 3 (e.g. a photodiode).
Indirect X-ray conversion therefore includes for example the combination of a scintillator layer (e.g. Gd2O2S or CsI with different doping agents such as terbium, thallium, europium, etc.; layer thicknesses typically 0.1-1 mm) and a photodetector (preferably a photodiode). The emission wavelength of the scintillator light as a result of X-ray conversion in this case overlaps with the spectral sensitivity of the photodetector.
In the case of direct X-ray conversion, the X-ray radiation is for example once again converted directly into electron-hole pairs and these are read out electronically (e.g. amorphous Se). Direct X-ray conversion into selenium is typically performed using layers up to 1 mm thick which are biased in the kV range (electrical fields up to 10 V/μm). Whereas indirectly converting detectors have become established as the norm, in particular because they can be produced easily and cost-effectively, direct converters generally possess a significantly better resolution capacity.
Many applications of organic electronics (e.g. organic light-emitting diode, organic light-emitting electrochemical cell, organic photovoltaics, organic field effect transistor or organic photodetector), such as e.g. detectors, for example X-ray detectors, are currently realized in process engineering terms by way of either physical gas phase or wet-chemical coating or printing methods, wherein said methods can be used for example for building the respective component architectures. In this regard gas phase deposition is used principally for organic small molecules, and wet-chemical processing both for small organic molecules and for polymers.
In this case gas phase deposition generally requires a complex and expensive process engineering solution, whereas wet-chemical depositions usually make the use of solvents, additives and/or dispersants necessary, which can detrimentally affect the components and/or necessitate heightened and cost-intensive safety measures, protective enclosures and personnel training programs on account of the hazardous nature of the substances added.
For many applications there is also a requirement for layers having homogeneous layer thicknesses of several 10 to several 100 μm, such as e.g. absorbing layers in gamma-ray and/or X-ray detectors, during the production of which by way of the above methods material losses and/or material damage may occur or special complex and expensive manufacturing measures are necessary.
In order to fabricate thicker layers, the production of detectors, in particular X-ray detectors, via dry phase deposition is proposed in DE 10 2013 226 339, DE 10 2014 225 543 and DE 10 2014 225 541.
Furthermore, methods are described in DE 10 2013 226 338, DE 10 2014 212 424, DE 10 2013 226 339 and DE 10 2014 203 685 which make provision in a first step for the production of core-shell powders and in a second step for the compression of the powders to form a homogeneous film. Said powders consist of particles that have an envelope composed of organic semiconductor materials.
In addition, the use of perovskites, e.g. lead iodide perovskites, in detector layers is disclosed in DE 10 2014 225 543 and DE 10 2014 225 541.
It is additionally known from the literature that lead iodide perovskites feature an ambipolar transport, with a longer diffusion constant for electrons than for holes, as described in Giorgi et al., Small Photocarrier Effective Masses Featuring Ambipolar Transport in Methylammonium Lead Iodide Perovskite: A Density Functional Analysis, Phys. Chem. Lett., 2013, 4 (24), pp. 4213-4216. With increasing layer thicknesses, the ambipolar transport and the unbalanced diffusion constant of the charge carriers can lead to an increase in recombination losses.
In this case the high electrical conductivity and high charge carrier mobility of the perovskites can have a positive effect in terms of the charge carrier extraction from the absorber layer, which can enable an improvement in efficiency, although conversely this can also lead to leakage currents and, as a result thereof, increased dark currents, which can limit the dynamic range of the X-ray detectors. On the other hand, the dynamic range of an X-ray detector can be increased by reducing the dark currents.
A reduction in dark currents can be achieved for example by a use of intermediate layers, as a result of which p-i-n structures can be formed. The intermediate layers, also called interlayers, can be deposited for example from the liquid phase or from the gas phase. Intermediate layers may consist for example of organic and inorganic conductors or semiconductors or comprise such. An example of such p-i-n structures is disclosed in Liu et al., Efficient planar heterojunction perovskite solar cells by vapor deposition, Nature 2013, Vol. 501, 397. As described there, the efficiency of solar cells is increased via gas phase deposition by comparison with liquid phase deposition. It is conceivable in this case that the diffusion constant of the charge carriers is greater in vapor-deposited perovskites than in liquid-processed ones.
Once a perovskite lattice has been formed, the transport properties of the charge carriers are usually dependent on the material and on the crystallinity of the layer. Thus, for example, the diffusion length of electrons and holes in lead iodide perovskites (CH3NH3PbI3) amounts to ˜1 μm, as demonstrated by Stranks et al., Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber, Science, 2013 Oct. 18; 342(6156):341-4. doi: 10.1126/science.1243982.
In addition to an absorption of visible light and X-ray radiation, a material crystallized in a perovskite lattice layer also exhibits good electrical conductivity of the generated charge carrier pairs and high mobility of e.g. up to 50 cm2/Vs. Thus, for example, a high power conversion efficiency of up to 19.3% can be obtained with a “perovskite” solar cell (solar cell produced via a material mixture crystallizing in the perovskite lattice) (Science. 2014 Aug. 1; 345(6196):542-6. doi: 10.1126/science.1254050.
Photovoltaics. Interface engineering of highly efficient perovskite solar cells. Zhou H, Chen Q, Li G, Luo S, Song T B, Duan H S, Hong Z, You J, Liu Y, Yang Y). This efficiency makes perovskites appear an interesting proposition for the detection of high-energy radiation such as gamma and/or X-ray radiation. However, in order to ensure adequate X-ray absorption, for example, large layer thicknesses of e.g. 10 μm up to 1 mm are required.
Doped perovskites and their use in optoelectronic devices are also disclosed in EP 2 942 826 A.